**2. Synergistic effect in SF6 gas mixtures**

*Modern Applications of Electrostatics and Dielectrics*

breakdown voltage of gas mixture is *U*m, and *K* is the mixing ratio of gas 1 in the gas mixture. For super synergistic effect, *U*m > *U*1 and *U*m > *U*2 exist in some mixing ratios, as shown in curve a; for synergistic effect, *U*m > *U*1 + *U*2, as shown in curve b; for linear relationship, *U*m = *U*1 + *U*2, as shown in curve c; for negative synergistic

At first, researchers studied the synergistic effect of SF6 gas mixture in order to solve the problems of high liquefaction temperature, sensitivity to electric field, and the high price of SF6 gas [7]. However, with the deepening understanding of SF6, scholars have found that SF6 is a strong greenhouse gas. It is estimated that the global annual production of SF6 gas is more than 20,000 tons and 80% of SF6 gas produced globally each year is used in the power industry. Although SF6 has many advantages, its greenhouse effect on the earth cannot be ignored. The global warming potential (GWP) value of SF6 gas is 23,900; it means that the emission of 1 kg SF6 is equivalent to the emission of 23,900 kg of CO2. What is more serious is that SF6 has a very stable chemical property, which is difficult to decompose after it spreads to the outside environment, and can exist for up to 3200 years. The environmental impact and

Affected by climate change, more and more international cooperation has been carried out to reduce greenhouse gas emissions, so as to curb global climate change and maintain the sustainable development of the environment. In the Kyoto protocol of the United Nations framework convention on climate change signed in Kyoto, Japan, in 1997, SF6 has been clearly regulated as one of the six greenhouse gases and requires developed countries to freeze and reduce the total greenhouse gas emissions [11]. It means that the use of SF6 in the industrial field will be increasingly restricted and pressured. Therefore, it is an urgent task to study a new gas insulation

Although at present there have been many studies about alternative SF6 gas, no gas can thoroughly replace SF6 gas in the form of a single gas; they all have to be mixed with buffer gas for industrial application. The reasonable use of synergistic effect can effectively improve dielectric strength of gas mixtures and reduce the use of insulation gas, so in this paper, the research progress and methods of synergistic effect with gas mixtures are introduced; the prospect and the difficulties in the field were also discussed. This paper is expected to provide help and reference for future

greenhouse effect generated by SF6 will continue to accumulate [8–10].

effect, *U*m < *U*1 + *U*2, as shown in curve d in **Figure 1**.

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scheme to replace SF6.

**Figure 1.**

*Types of synergistic effect.*

research on synergistic effect.

In view of the synergistic effect and insulation strength of SF6 mixture, scholars studied and analyzed it through theoretical calculation and experimental research. **Figure 2a** is a simple and intuitive calculation method proposed by Wieland et al. for the insulation strength of SF6 gas mixture, and **Figure 2b** is the comparison of the calculation results with the actual values and weighted values [12].

Christophorou et al. thought that the preferred gas mixture should include effective electron-attaching gas and/or electron-slowing down gas [13]. The attachment cross section of electron-attaching components should be as wide as possible, or the attachment cross section of different gases is in different energy interval, so the attachment cross section of gas mixtures is also wide. The effect of electron-slowing down gas is to slow down the free electrons, making them easier to attach and reducing secondary ionization. Based on this theory, they believed that when an electronegative gas and a gas with a large dipole moment are mixed, the synergistic effect and insulation strength of the gas mixture will be better. **Figure 3** shows the experimental results of SF6 gas mixtures with CF4, CHF3, and 1,1,1-CH3CF3 gas, and the electric dipole moments of these three gases are 0, 1.65D, and 2.32D, respectively.

From the figure, it can be seen that the breakdown voltage curve of SF6-CF4 gas mixtures shows almost a linear trend, and the electric dipole moments of CF4 is 0. When it comes to SF6-CHF3 gas mixture, as the content of SF6 gas increases, the breakdown voltage of the gas mixtures does not increase in a straight line, and there is synergistic effect that occurs. The electric dipole moments of 1,1,1-CH3CF3 gas is 2.32D, which is the one with the largest electric dipole moment among the three gases; from **Figure 3c** it can be seen that the synergistic effect of this gas mixture is the most pronounced.

Okubo et al. investigated the partial discharge (PD) and breakdown characteristics of SF6-N2 gas mixtures in order to analyze the relationship between electronegativity, additive gases, and the insulation strength [14]. They believed that the synergistic effect of gas mixture is related to the change of discharge form. **Figure 4** shows the

#### **Figure 2.**

*Experimental results of SF6 gas mixtures. (a) Wieland calculation method, (b) Results comparison of different methods.*

**Figure 3.** *Experimental results of SF6 gas mixtures. (a) SF6-CF4, (b) SF6-CHF3, (c) SF6-1,1,1-CH3CF3.*

**Figure 4.** *Impulse PD in SF6-N2 gas mixture. (a) 0.1MPa, (b) 0.2MPa, (c) 0.3MPa.*

impulse PD of SF6-N2 gas mixture at different pressure. The needle-plate electrode was used in the experiment, and the content of SF6 was 10% and N2 was 90%.

It can be seen that with the change of gas pressure, the development of impulse PD has changed. When the gas pressure is 0.1 MPa, as shown in **Figure 5a**, the brushlike partial discharge occurred around the needle electrode, and it can be thought of as streamer discharge. When the pressure increases to 0.2 MPa, it can be seen from **Figure 5b** that the development of discharge process becomes longer; streamer discharge turns into leader discharge. As the pressure continues to increase, when the pressure is 0.3 MPa, the PD type of the gas mixture is still the leader discharge, and the path of discharge development becomes shorter with the increase of the pressure.

Yamada et al. studied the insulation properties of a kind of gas mixture containing ultra-dilute SF6 gas [15]. It has been found that trace SF6 has a significant effect on the streamer discharge of the gas mixture. As the SF6 content increases, the number of the discharge channels decreased significantly, and the number of channels that can reach the plane electrode also reduced, as shown in **Figure 4**. Except for the effect on discharge characteristics, the results show that the breakdown voltage and the PD voltage of SF6/N2 gas mixture have a significant synergistic effect. Yamada T thought that the addition of trace SF6 inhibits the development of streamer discharge process, which leads to synergistic effect.

Osmokrovic et al. conducted an in-depth study on the synergistic effect of SF6/N2 under impulse voltage. The experimental results show that the synergistic

**111**

800 kv/ms, respectively.

*PD characteristics in SF6-N2 gas mixture.*

**Figure 5.**

the synergistic effect of SF6 gas mixture.

*Research Progress on Synergistic Effect between Insulation Gas Mixtures*

effect of SF6/N2 gas mixture related to the rising rate of the impulse voltage. As the impulse voltage rise rate increases, the synergistic effect is gradually weakened. The synergistic effect of SF6 gas mixture is very weak and almost completely disappears under some impulse voltage with very high rise rate [16]. The insulation characteristics of SF6 and SF6/N2 gas mixture under impulse voltage with different rise rate are shown in **Figure 5**. The rise rates of shock voltage in **Figure 6a–c** are 1, 50 and

Based on the above phenomenon, Osmokrovic proposed that after adding N2 to the SF6 gas, electrons can make N2 vibration and rotation dynamics excited or dissociated, this process will make electrons lose energy, and the effective temperature decreases, to realize the modulation of electron energy spectrum and increase the probability that the electrons are captured. The rise rate of impulse voltage has influence on the modulation of the electron energy spectrum, which in turn affects

Hayakawa et al. studied the generation and development of PD characteristics of SF6/N2 gas mixture under positive lightning impulse [17]. PD and breakdown characteristics with different SF6 content are shown in **Figure 7**. Hayakawa proved through the film of the streak camera and ICCD that the discharge process in SF6/ N2 gas mixture did change with the increase of gas pressure. Streamer discharge and leader discharge are the two types of PD process in SF6/N2 gas mixture, which all have relationship with gas pressure and SF6 content. With the increase of gas pressure and the content of SF6 gas, the leader discharge process gradually takes the

leading position, and the streamer discharge process of gradually weakens. Chen studied the discharge characteristics of SF6/N2 gas mixture under DC voltage and lightning impulse in extremely uneven electric field. The experimental

*DOI: http://dx.doi.org/10.5772/intechopen.90705*

*Research Progress on Synergistic Effect between Insulation Gas Mixtures DOI: http://dx.doi.org/10.5772/intechopen.90705*

*Modern Applications of Electrostatics and Dielectrics*

impulse PD of SF6-N2 gas mixture at different pressure. The needle-plate electrode was used in the experiment, and the content of SF6 was 10% and N2 was 90%.

*Impulse PD in SF6-N2 gas mixture. (a) 0.1MPa, (b) 0.2MPa, (c) 0.3MPa.*

*Experimental results of SF6 gas mixtures. (a) SF6-CF4, (b) SF6-CHF3, (c) SF6-1,1,1-CH3CF3.*

streamer discharge process, which leads to synergistic effect.

Osmokrovic et al. conducted an in-depth study on the synergistic effect of SF6/N2 under impulse voltage. The experimental results show that the synergistic

It can be seen that with the change of gas pressure, the development of impulse PD has changed. When the gas pressure is 0.1 MPa, as shown in **Figure 5a**, the brushlike partial discharge occurred around the needle electrode, and it can be thought of as streamer discharge. When the pressure increases to 0.2 MPa, it can be seen from **Figure 5b** that the development of discharge process becomes longer; streamer discharge turns into leader discharge. As the pressure continues to increase, when the pressure is 0.3 MPa, the PD type of the gas mixture is still the leader discharge, and the path of discharge development becomes shorter with the increase of the pressure. Yamada et al. studied the insulation properties of a kind of gas mixture containing ultra-dilute SF6 gas [15]. It has been found that trace SF6 has a significant effect on the streamer discharge of the gas mixture. As the SF6 content increases, the number of the discharge channels decreased significantly, and the number of channels that can reach the plane electrode also reduced, as shown in **Figure 4**. Except for the effect on discharge characteristics, the results show that the breakdown voltage and the PD voltage of SF6/N2 gas mixture have a significant synergistic effect. Yamada T thought that the addition of trace SF6 inhibits the development of

**110**

**Figure 3.**

**Figure 4.**

#### **Figure 5.** *PD characteristics in SF6-N2 gas mixture.*

effect of SF6/N2 gas mixture related to the rising rate of the impulse voltage. As the impulse voltage rise rate increases, the synergistic effect is gradually weakened. The synergistic effect of SF6 gas mixture is very weak and almost completely disappears under some impulse voltage with very high rise rate [16]. The insulation characteristics of SF6 and SF6/N2 gas mixture under impulse voltage with different rise rate are shown in **Figure 5**. The rise rates of shock voltage in **Figure 6a–c** are 1, 50 and 800 kv/ms, respectively.

Based on the above phenomenon, Osmokrovic proposed that after adding N2 to the SF6 gas, electrons can make N2 vibration and rotation dynamics excited or dissociated, this process will make electrons lose energy, and the effective temperature decreases, to realize the modulation of electron energy spectrum and increase the probability that the electrons are captured. The rise rate of impulse voltage has influence on the modulation of the electron energy spectrum, which in turn affects the synergistic effect of SF6 gas mixture.

Hayakawa et al. studied the generation and development of PD characteristics of SF6/N2 gas mixture under positive lightning impulse [17]. PD and breakdown characteristics with different SF6 content are shown in **Figure 7**. Hayakawa proved through the film of the streak camera and ICCD that the discharge process in SF6/ N2 gas mixture did change with the increase of gas pressure. Streamer discharge and leader discharge are the two types of PD process in SF6/N2 gas mixture, which all have relationship with gas pressure and SF6 content. With the increase of gas pressure and the content of SF6 gas, the leader discharge process gradually takes the leading position, and the streamer discharge process of gradually weakens.

Chen studied the discharge characteristics of SF6/N2 gas mixture under DC voltage and lightning impulse in extremely uneven electric field. The experimental

**Figure 6.**

*Insulation characteristics of SF6 and SF6/N2 gas mixture under different rising rate impulse voltages. (a) 1kV/ms, (b) 50kv/ms, (C) 800kV/ms.*

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mixture [19].

**Figure 7.**

*(c) SF620–N280%, (d) pure SF6.*

*Research Progress on Synergistic Effect between Insulation Gas Mixtures*

results show that there was a significant decrease of breakdown voltage with the increase of gas pressure under DC voltage and the abnormal discharge characteristics pressure range of SF6/N2 gas mixture is larger than SF6 gas. This phenomenon leads to the breakdown voltage of SF6/N2 gas mixture is higher than that of SF6 gas. So under this abnormal range, positive synergistic effect occurs. However, under lighting impulse voltage, no abnormal discharge phenomenon was found, but posi-

*PD and breakdown characteristics with different SF6 content. (BDV50, breakdown voltage; PDIV50, 50% probability PD inception voltage; LOV, leader discharge onset voltage). (a) SF65–N295%, (b) SF610–N290%,* 

Tagashira et al. believe that synergistic effects can be divided into three categories: *a*-synergistic effect (SF6 + SiH4), *η*-synergistic effect (SF6 + c-C4F8), and γ-synergistic effect (N2 + CH4). The research found that the curves of SF6/SiH4, SF6/c-C4F8, SF6/C3F6 gas mixtures with SF6 gas content all decreased first and then increased, that is, the curve had a minimum point. For the curve of SF6/SiH4 gas mixture, the falling part of the curve is due to the decrease of ionization coefficient *α*, that is, *α*-synergistic effect, and for the curve of SF6/c-C4F8 gas mixture, the rising part is caused by the increase of attachment coefficient *η* and that is *η*-synergistic effect. In addition to these two synergistic effects, they also proposed a synergistic effect of γ on the secondary ionization coefficient of N2 + CH4 gas

tive synergistic effect still existed, as shown in **Figure 8** [18].

*DOI: http://dx.doi.org/10.5772/intechopen.90705*

*Research Progress on Synergistic Effect between Insulation Gas Mixtures DOI: http://dx.doi.org/10.5772/intechopen.90705*

#### **Figure 7.**

*Modern Applications of Electrostatics and Dielectrics*

**112**

**Figure 6.**

*(a) 1kV/ms, (b) 50kv/ms, (C) 800kV/ms.*

*Insulation characteristics of SF6 and SF6/N2 gas mixture under different rising rate impulse voltages.* 

*PD and breakdown characteristics with different SF6 content. (BDV50, breakdown voltage; PDIV50, 50% probability PD inception voltage; LOV, leader discharge onset voltage). (a) SF65–N295%, (b) SF610–N290%, (c) SF620–N280%, (d) pure SF6.*

results show that there was a significant decrease of breakdown voltage with the increase of gas pressure under DC voltage and the abnormal discharge characteristics pressure range of SF6/N2 gas mixture is larger than SF6 gas. This phenomenon leads to the breakdown voltage of SF6/N2 gas mixture is higher than that of SF6 gas. So under this abnormal range, positive synergistic effect occurs. However, under lighting impulse voltage, no abnormal discharge phenomenon was found, but positive synergistic effect still existed, as shown in **Figure 8** [18].

Tagashira et al. believe that synergistic effects can be divided into three categories: *a*-synergistic effect (SF6 + SiH4), *η*-synergistic effect (SF6 + c-C4F8), and γ-synergistic effect (N2 + CH4). The research found that the curves of SF6/SiH4, SF6/c-C4F8, SF6/C3F6 gas mixtures with SF6 gas content all decreased first and then increased, that is, the curve had a minimum point. For the curve of SF6/SiH4 gas mixture, the falling part of the curve is due to the decrease of ionization coefficient *α*, that is, *α*-synergistic effect, and for the curve of SF6/c-C4F8 gas mixture, the rising part is caused by the increase of attachment coefficient *η* and that is *η*-synergistic effect. In addition to these two synergistic effects, they also proposed a synergistic effect of γ on the secondary ionization coefficient of N2 + CH4 gas mixture [19].

**Figure 8.** *Abnormal discharge phenomenon in SF6/N2 and SF6.*

Takuma et al. studied the synergistic effect of gas mixtures such as SF6/N2, CCl2F2/N2, etc. They assumed that the effective ionization coefficient of the gas mixture is equal to the sum of the coefficients of the two component gases multiplied by their respective partial pressure ratios, suggesting an empirical formula for breakdown voltage of SF6/N2 gas mixture under slight uneven electric field [20]: \_*k*

$$U\_m = U\_2 + \frac{k}{k + C(1 - k)}(U\_1 - U\_2) \tag{1}$$

where *U*1 and *U*2 are the breakdown voltage of component gas 1 and gas 2, *U*<sup>m</sup> is the breakdown voltage of gas mixture (*U*1 > *U*2), *k* is the partial pressure ratio of component gas 1, and *C* is the synergistic effect coefficient, which is independent of the partial pressure ratio *k*. When Constant *C* = 0.08, the calculated and experimental values of the SF6/N2 gas mixture by this formula are shown in **Figure 5**. The breakdown voltage values are basically the same, which can reflect the synergistic effect of SF6 gas mixture.

According to formula (1), the formula for calculating the synergy coefficient proposed by Takuma is

posed by läkuma is 
$$C = \frac{k\left(U\_1 - U\_M\right)}{\left(1 - k\right)\left(U\_M - U\_2\right)}\tag{2}$$

**115**

**Figure 10.**

*Research Progress on Synergistic Effect between Insulation Gas Mixtures*

*h* =

*curvatures of needle electrodes). (a) R = 2 mm, (b) R = 1 mm.*

*Breakdown voltage differences between N2 and SF6/N2.*

negative synergistic effect" appears, then value of the coefficient *C* is still less than 0, so the various synergistic effects of the gas mixture cannot be clearly distin-

(*Um* <sup>−</sup>*U*2) <sup>−</sup> *<sup>k</sup>*(*U*<sup>1</sup> <sup>−</sup>*U*2) \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ 0.5(*<sup>U</sup>*<sup>1</sup> <sup>+</sup>*U*2)

*h* = 0 *k* = 0 or 1

*Coefficient* h *for SF6/N2 gas mixture under lightning impulse with different needle-plane electrode (*R *is radius* 

where *U*1, *U*2, *U*m, and *k* in Eq. 3 have the same meaning as those in Eq. 2. When *h* > 0, it means that gas mixture has synergistic effect, and when *h* < 0, it represents that the gas mixture has negative synergistic effect. The relationship between the

Guo et al. introduced a normalization coefficient *h* to investigate the synergistic effect of SF6/N2 gas mixture under lightning impulse. The definition of coefficient h

0 < *k* < 1

(3)

*DOI: http://dx.doi.org/10.5772/intechopen.90705*

guished by formula (2).

is as follows:

**Figure 9.**

As can be seen from Eq. (2), when *C* = 1, the breakdown voltage of the gas mixture is equal to the weighting value of breakdown voltage of the two components according to the mixing ratio. That is, the breakdown voltage of the gas mixture exhibits a linear relationship that increases as the mixing ratio of the component gas increases. When 0 < *C* < 1, the breakdown voltage of the gas mixture reflects synergistic effect phenomenon, and the smaller the value of *C*, the more significant the nonlinear increase of the breakdown voltage of the gas mixture, which means synergistic effect becomes more significant. When *C* = 0, the breakdown voltage of the gas mixture equals to the breakdown voltage of gas 1, which means *U*M = *U*1.

If all the situations of synergistic effect, i.e., positive synergistic effect, synergistic effect, and negative synergistic, are considered at the same time, then the above formula is no longer applicable. Assuming a positive synergistic effect of the gas mixture, it can be seen from the calculation that the coefficient *C* < 0 under positive synergistic. When the breakdown voltage of the gas mixture is lower than all the breakdown voltage of the component gas, that is, the phenomenon of "super *Research Progress on Synergistic Effect between Insulation Gas Mixtures DOI: http://dx.doi.org/10.5772/intechopen.90705*

negative synergistic effect" appears, then value of the coefficient *C* is still less than 0, so the various synergistic effects of the gas mixture cannot be clearly distinguished by formula (2).

Guo et al. introduced a normalization coefficient *h* to investigate the synergistic effect of SF6/N2 gas mixture under lightning impulse. The definition of coefficient h is as follows:

$$\begin{aligned} \text{true under lightring impulse. The definition of } \stackrel{\circ}{\text{coefficient}}\\ h &= \frac{\{U\_m - U\_2\} - k(U\_1 - U\_2)}{0.5(U\_1 + U\_2)} & \quad 0 < k < 1\\ h &= 0 & \quad k = 0 \text{ or } 1 \end{aligned} \tag{3}$$

where *U*1, *U*2, *U*m, and *k* in Eq. 3 have the same meaning as those in Eq. 2. When *h* > 0, it means that gas mixture has synergistic effect, and when *h* < 0, it represents that the gas mixture has negative synergistic effect. The relationship between the

#### **Figure 9.**

*Modern Applications of Electrostatics and Dielectrics*

*Abnormal discharge phenomenon in SF6/N2 and SF6.*

Takuma et al. studied the synergistic effect of gas mixtures such as SF6/N2, CCl2F2/N2, etc. They assumed that the effective ionization coefficient of the gas mixture is equal to the sum of the coefficients of the two component gases multiplied by their respective partial pressure ratios, suggesting an empirical formula for breakdown voltage of SF6/N2 gas mixture under slight uneven electric field [20]:

\_*k*

*k* + *C*(1 − *k*)

where *U*1 and *U*2 are the breakdown voltage of component gas 1 and gas 2, *U*<sup>m</sup> is the breakdown voltage of gas mixture (*U*1 > *U*2), *k* is the partial pressure ratio of component gas 1, and *C* is the synergistic effect coefficient, which is independent of the partial pressure ratio *k*. When Constant *C* = 0.08, the calculated and experimental values of the SF6/N2 gas mixture by this formula are shown in **Figure 5**. The breakdown voltage values are basically the same, which can reflect the synergistic

According to formula (1), the formula for calculating the synergy coefficient

As can be seen from Eq. (2), when *C* = 1, the breakdown voltage of the gas mixture is equal to the weighting value of breakdown voltage of the two components according to the mixing ratio. That is, the breakdown voltage of the gas mixture exhibits a linear relationship that increases as the mixing ratio of the component gas increases. When 0 < *C* < 1, the breakdown voltage of the gas mixture reflects synergistic effect phenomenon, and the smaller the value of *C*, the more significant the nonlinear increase of the breakdown voltage of the gas mixture, which means synergistic effect becomes more significant. When *C* = 0, the breakdown voltage of the gas mixture equals to the breakdown voltage of gas 1, which means *U*M = *U*1. If all the situations of synergistic effect, i.e., positive synergistic effect, synergistic effect, and negative synergistic, are considered at the same time, then the above formula is no longer applicable. Assuming a positive synergistic effect of the gas mixture, it can be seen from the calculation that the coefficient *C* < 0 under positive synergistic. When the breakdown voltage of the gas mixture is lower than all the breakdown voltage of the component gas, that is, the phenomenon of "super

(*U*<sup>1</sup> − *U*2) (1)

(2)

*Um* = *U*<sup>2</sup> +

*<sup>C</sup>* = *k*(*U*<sup>1</sup> <sup>−</sup>*UM*) \_\_\_\_\_\_\_\_\_\_\_\_\_ (1 <sup>−</sup> *<sup>k</sup>*)(*UM* <sup>−</sup>*U*2)

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effect of SF6 gas mixture.

**Figure 8.**

proposed by Takuma is

*Coefficient* h *for SF6/N2 gas mixture under lightning impulse with different needle-plane electrode (*R *is radius curvatures of needle electrodes). (a) R = 2 mm, (b) R = 1 mm.*

**Figure 10.** *Breakdown voltage differences between N2 and SF6/N2.*

coefficient *h* of SF6/N2 and *k* under lightning impulse with different needle-plane electrodes is shown in **Figure 9** [21].

The analysis results show that under the action of negative lightning impulse voltage, the negative synergistic effect increases with the increase of gas pressure. The synergistic effect under the positive impact voltage decreases with the decrease of gas pressure. With the increase of the electric field inhomogeneity coefficient, the synergistic effect has a negative synergistic effect. The analysis of the development process of the flow discharge shows that there are three reasons for the negative synergistic effect: similar flow corona starting voltage, different space charge effects, and different N2 and SF6/N2 mixed gas discharge forms. The difference of breakdown voltage between N2 and SF6/N2 gas mixture is shown in **Figure 10**. *r*sp represents the range of space charge, Δ*U*SP is the influence of space charge on breakdown voltage, *U*st is the streamer corona onset voltage, and *U*b = *U*st + Δ*U*SP.
